The present invention relates to an apparatus and method for measuring the level of oxygen in a fluid stream.
Presently available devices for measuring dissolved oxygen in situ are subject to certain disabilities, the major disability being the requirement of calibrating the instrument or a daily basis.
It has been recognized that an oxygen sensor can be used with a measuring process known as chronoamperoemetry of pulsed potential voltammetry, first described by Lilley et. al., J. Electronanal. Chem., 23 (1969) 426-429; see also Smart et. al., In Situ Voltammetric Membrane Ozone Electrode, Anal. Chem. Vol. 51, No. 14, December '79, p. 2315-2319, and Non-Steady State Measurement, "The Measurement of Dissolved Oxygen", 1979.
In Lilley, the polarizing potential is applied as a pulse rather than a steady level. The pulse perturbs the sensor from equilibrium generating a large current transient which can be related to the oxygen partial pressure of the sample. Lilley recognized that this process had the important advantage over steady state measurements of increasing sensitivity and eliminating the requirement for stirring the sample solution as long as the current was read within four seconds of pulse application.
In spite of this teaching by Lilley in 1969, to date it is not believed that commercially available oxygen sensors embody the teachings of Lilley, but rather, that they use steady state measurements which measurements are subject to drift and the solution in which the electrode is immersed must be continually stirred to refresh the solution at the electrode surface.
The present invention provides a method and apparatus which electrochemically measures the quantity of a species in a fluid stream. More particularly, my invention provides a device wherein the acquisition, storage and generation of information corresponding to the oxygen level in a fluid stream is handled in a control module designed especially for that purpose.
Broadly, the invention comprises a sensor immersed in a fluid stream, the sensor specific for the species in the fluid stream to be measured, a switch to energize the sensor for a duration (d) and to prevent energization of the sensor for an interval (i); a circuit to real time average the signal from the sensor; an analog acquisition board and a computer adapted to control the activation of the switch, data acquisition and storage and all timing functions. In a preferred embodiment, temperature measurements are also derived from the sensor.
My invention achieves major economies in hardware using a microcomputer to control all important process parameters such as pulse duration, delay and repetition rate. Real time signal averaging is used to reduce the noise level and thereby improve precision. My device, when used for environmental sampling, does not require temperature salinity or pressure compensation circuitry.
The invention is particularly suitable for applications requiring long term measurements in environments where it is impracticable to service and recalibrate the sensor frequently for example, industrial processes, such as fermentors, secondary sewage treatment plants and oceanographic research. The major advantages of my invention are increased precision (changes as as small as 0.3 umolar can be detected, one time calibration) and the elimination of a stirring or flow rate requirement. The sensor is interfaced to the microprocessor, a complete record of time, temperature and oxygen concentration may be stored and made available for printout, plotting and statistical analysis.